HP 8712ES User Manual page 264

Calibrating for Increased Measurement Accuracy
Calibration Choices
NOTE When toggled on, the Isolation on OFF softkey allows you to remove
systematic errors caused by crosstalk. To select it, press CAL
User 2-Port Isolation on OFF User 2-Port . A two-port
calibration always measures the same parameters, regardless of the
setting of Isolation on OFF . Therefore, no further measurements of
calibration standards are required when you enable the isolation
calibration feature. When toggled ON, this feature will change the
analyzer's calculations before displaying the data. Crosstalk can be a
problem with high-isolation devices and high-dynamic range devices.
Since this isolation calibration adds noise to the error model, you should
only select it if it is really needed.
Transmission Calibration Interpolation
Widening the frequency span after performing any of these calibrations
will invalidate them, and restore the default calibration. You may
narrow the span and the analyzer will interpolate between calibration
points for the narrower span.
Note that after you have performed a calibration, a "C" appears in the
upper right hand corner of the display. This "C" indicates that a
user-defined cal is in use. If you change to a narrower span, note that the
"C" changes to "C?", indicating the analyzer is now interpolating between
calibrated measurement points. The "C?" notation also appears when
other system parameters, such as power, number of points, or sweep
time, have changed.
6-18 ES User's Guide